1. Field of the Invention
The present invention relates to a process for converting water-sensitive ceramic powders into a free-flowing granulated powder. In particular the invention relates to a procedure for processing aluminum nitride powder.
2. Description of Related Art
In the production of shaped technical ceramic parts the ceramic powder is first processed before it is formed into a green body and the green body is sintered.
Fine powders are prone to agglomerate as a result of surface forces with or without chemical bonding, making the processing into a green body difficult or impossible, since the agglomerated powder is not free-flowing and this leads to inhomogeneities in the microstructure of the sintered ceramic. Hence every production process for shaped ceramic parts requires specific powder processing in which the powder is deagglomerated. A controlled agglomeration can subsequently be carried out in the powder processing to obtain a free-flowing granulated powder with a desired specification. Free-flowing granulated powders are used, for example, if the forming of the green body is by pressing.
If the green body is densified by pressureless liquid phase sintering, the processing of the powder also serves to distribute a sintering aid homogeneously in the ceramic powder.
One way of powder processing comprises wet milling in water with subsequent spray drying to give a free-flowing granulated powder. In the case of some ceramic powders this type of powder processing with water can lead to undesirable changes in the material. In the case of aluminum oxide powder, for example, the surface can be so changed by water that the processing behavior of the material in injection molding is unfavorably affected. A particular problem arises in processing aluminum nitride powder with water, since aluminum nitride powder is very sensitive to hydrolysis. The hydrolysis leads to a substantial reduction in the thermal conductivity of the aluminum nitride ceramic. Since aluminum nitride ceramic, because of its very good thermal conductivity, is finding increasing application in electronics, for example as substrate material in microelectronics, hydrolysis must be prevented in the production process of shaped parts made of aluminum nitride powder.
One way of preventing the hydrolysis of aluminum nitride powder during processing comprises the use of organic dispersing media for powder processing. However organic dispersing media have the disadvantage that they are harmful to health and/or the environment, or form explosive mixtures with air.
Another possibility is to coat the aluminum nitride powder in order to avoid direct contact between powder and water during powder processing. However, the coated powder, coated for example with polymers or fatty acids, is then barely dispersible in water, if at all. Other coatings can give problems during burnout or sintering, if relatively large amounts of these additives need to be removed.
EP 377 701 describes a process for wet milling of ceramic powders, in particular zirconium dioxide, in which the milling is carried out in the presence of ammonia or alkylamines with boiling points of not more than 100.degree. C. These dispersants are added to the powder suspended in water in sufficient quantity for the mill slip to have a sufficiently fluid consistency. In the case of ammonia and gaseous alkylamines, the dispersants are added in the form of aqueous solutions.